Airframes designed for intentionally induced and maintained roll rates about their longitudinal axes have significant practical advantages over roll stabilized airframes. The rolling airframe concept has been applied to air and surface launched missiles. Such missiles may be spun up initially by the launcher and then utilize canted control surfaces to maintain a roll rate of approximately 5 to 15 revolutions per second. With such a roll rate, it is possible to utilize a single control plane to guide the missile in all three earth related axes. In a typical instance, the control system utilizes a single pair of variable incidence control surfaces to pitch the missile about the control plane at a selected instantaneous rotational orientation upon command from a guidance command signal. Thus, with such a missile operating in a level flight attitude, to cause the missile to climb, a guidance command signal must vary in amplitude at a frequency equal to the roll rate of the missile. For the example given, the guidance command signal would be a generally sinusoidal waveform that would induce pitch-up as the control plane of the vehicle approaches earth vertical and pitch-down after the control plane rotates and nears a one-half revolution from pitch-up, thereby producing a vertical angle of attack. The angle of attack produces body lift and alters the missile course from a horizontal to a climbing course. Similarly, a course change to the right would be effectuated by a sinusoidal signal displaced 90.degree. from the signal required for a vertical course change.
Because the rolling airframe missile has a simplified control system, a reduction in cost and increase in reliability is realized over stablized airframe. However, an effective and practical autopilot has not previously been developed for rolling airframe missiles, and therefore, all such designs to date have incorporated open-loop control. The systems utilized are designated open-loop in that they utilize a control demand that is not modified by feed-back as to the actual changes resulting in the missile flight parameters. Such open-loop control requires that the missile process a high degree of aerodynamic stability. The stability required is similar to that associated with low performance manned aircraft to cause the aircraft to return to a straight and level flight attitude after control pressures are removed. The stability requirement dictates that the center of pressure for the missile must be located aft of the center of gravity. Such a static stability inherently reduces the maneuverability of the airframe in that the control surfaces must first offset the stability generated forces to achieve a given angle of attack. Further, the lack of a feedback system can result in over-shoot of a selected maneuver limit. Therefore, all of the airframe components and operating parameters in such a system must be conservative to insure the structural integrity of the airframe during operating conditions.
Conventional autopilot technology is not readily adaptable to the rolling airframe environment. In the conventional autopilot, fully stabilized flight and controlled maneuvers are obtained by detecting acceleration, rate and/or attitude with respect to the three related axes (i.e., ptich, yaw, roll). Such an autopilot then commands the airframe via the three axes control system to appropriately correct for detected errors in the flight path. The object of such an autopilot in straight and level flight is to produce a zero velocity of the airframe about each of the three earth related axes. Accordingly, the sensors utilized must be accurate and not have a DC offset at zero velocity. Such autopilots are sensitive and utilize complex mechanisms and therefore, are expensive and relatively unreliable. The complexity of a conventional autopilot is further increased by attempts to adapt it to a rolling airframe. In addition, many sensors suitable for use in the non-rolling environment cannot function effectively when subjected to continuous roll environments.
The lack of a practical autopilot for rolling airframes has limited their potential, particularly for high maneuverability applications. Therefore, it is desirable to have a rolling airframe autopilot capable of supplanting static aerodynamic stability, especially where such a rolling airframe autopilot is less complex than conventional non-rolling airframe autopilots.